Loops or Knots on the Underside of the Fabric

Another in the series of troubleshooting tips.  Tips are not in order of most common to least common and do not need to be performed in sequence.  Pick the easiest ones first and work to the more difficult ones.  Not all tips will apply to all machines.

The lower thread lies straight on the underside of fabric and the upper thread appears there in form of loops or small knots.

1. Cause: The tension of the upper thread is too weak or the tension of the lower thread is too tight.
 Solution: Tighten the tension of the upper thread, or loosen the tension of the lower thread, until the tensions of both threads are correctly balanced.

2. Cause: There are dirt, lint or pieces of thread between the Tension Discs.
 Solution: Take apart the upper tension, clean the Tension Discs thoroughly, then replace the upper tension and adjust it correctly.

3. Cause: The upper thread has cut deep grooves into the Tension Discs.
 Solution: Replace the defective Tension Discs with new ones.

4. Cause: The Bobbin Case is not threaded correctly. The Bobbin unwinds itself in the wrong direction.
 Solution: Thread the Bobbin Case correctly.

5. Cause: The head of the Tension Adjusting Screw of the Bobbin Case protrudes too much and catches and retards the upper thread.
 Solution: Adjust this screw correctly. If this condition continues to persist, smoothen the head of this screw with fine emery cloth or replace the defective screw with a new one.

6. Cause: The stitch hole in the Throat Plate or Feed Dog is too small.
 Solution: Replace the Throat Plate or Feed Dog with one that has a larger stitch hole.

7. Cause: The point of the Needle is bent over ("hooked point").
 Solution: Replace the defective Needle with a new one.

8. Cause: The Bobbin is not evenly wound. It unwinds itself irregularly.
 Solution: Replace the Bobbin with one that is wound evenly.

9. Cause: The Bobbin is damaged or bent and does not revolve freely and evenly.
 Solution: Replace the defective Bobbin with a new one.

10. Cause: Lint or dirt in the Bobbin Case prevents the Bobbin from revolving freely.
 Solution: Remove all dirt from this part. Clean and dry thoroughly before replacing in the machine.

11. Cause: The needle is not correctly timed in relation to the Loop Taker (Hook).
 Solution: Make certain that the Needle is pushed all the way up into the Needle Bar and held there firmly. Adjust the point of the Loop Taker in relation to the Needle. For "Timing" follow this general rule: The Needle must have risen about 3/32 of an inch from its lowest position at the moment the point of the Loop Taker is just at the center of the rising Needle. At this moment the point of the Loop Taker must be about 1/16 of an inch above the eye of the Needle.

12. Cause: There is insufficient clearance between the Bobbin Case Holder and the Bobbin Case Holder Position Bracket in horizontal axis Rotary Hook Machines. On vertical axis rotating hook types may be insufficient clearance between tab extending from Bobbin Case to the left and the notch at the underside of the Throat Plate.
 Solution: Turn the machine by hand and observe where the upper thread gets caught.
Increase the clearance, where required, by carefully removing material from the component parts of the machine (by means of fine emery cloth). Smoothen the repaired portions with crocus cloth. The heaviest thread to be used must pass freely between these parts.

13. Cause: The mechanism for releasing the upper thread tension is incorrectly adjusted. It releases the tension of the upper thread too early.
 Solution: Adjust this mechanism so that the tension of the upper thread is released only during the last moment of the upward motion of the Presser Bar Lifter.

14. Cause: The upper thread gets caught somewhere below the Throat Plate.
 Solution: Examine Hook and Bobbin Case with regard to rough spots which may catch the upper thread and retard it, thus causing the loops on the underside of the fabric. Eliminate all rough spots with the aid of an oilstone or with fine emery cloth, and then polish these portions with crocus cloth or on a buffing wheel.

15. Cause: The Thread Take-up Spring (Check Spring) is not adjusted and does not work properly.
 Solution: Adjust the Thread Take-up Spring until it has the correct tension and its upward motion is just finished at the moment the Needle enters the fabric.

Ed
Ed's Vintage Sewing Machine Shop